Add start_date parameter to the Issue REST API

What does this MR do and why?

The start_date field is already exposed in the API response and handled by the WorkItems::Callbacks::StartAndDueDate callback.

This adds the missing Grape parameter declaration so it can be set on create and update, and protects it behind the same set_issue_metadata permission check as due_date.

References

• #539195

Screenshots or screen recordings

# Create an issue with a start date
❯ curl --request POST \
  --header "PRIVATE-TOKEN: $GDK_PAT" \
  --url "http://gdk.test:3000/api/v4/projects/37/issues" \
  --data "title=New%20feature&start_date=2025-01-01"
{"id":723,"iid":3,"project_id":37,"title":"New feature","description":null,"state":"opened","created_at":"2026-05-27T01:24:05.041Z","updated_at":"2026-05-27T01:24:05.041Z","closed_at":null,"closed_by":null,"labels":[],"milestone":null,"assignees":[],"author":{"id":1,"username":"root","public_email":null,"name":"Administrator","state":"active","locked":false,"avatar_url":"https://www.gravatar.com/avatar/15bbd3f9e9852c9d6f0e82619c57f34c26004189add66e481c8524d6394ea445?s=80\u0026d=identicon","web_url":"http://gdk.test:3000/root"},"type":"ISSUE","assignee":null,"user_notes_count":0,"merge_requests_count":0,"upvotes":0,"downvotes":0,"start_date":"2025-01-01","due_date":null,"confidential":false,"discussion_locked":null,"issue_type":"issue","web_url":"http://gdk.test:3000/features/add-start-date-rest-api/-/work_items/3","time_stats":{"time_estimate":0,"total_time_spent":0,"human_time_estimate":null,"human_total_time_spent":null},"task_completion_status":{"count":0,"completed_count":0},"weight":null,"blocking_issues_count":0,"has_tasks":true,"task_status":"","_links":{"self":"http://gdk.test:3000/api/v4/projects/37/issues/3","notes":"http://gdk.test:3000/api/v4/projects/37/issues/3/notes","award_emoji":"http://gdk.test:3000/api/v4/projects/37/issues/3/award_emoji","project":"http://gdk.test:3000/api/v4/projects/37","closed_as_duplicate_of":null},"references":{"short":"#3","relative":"#3","full":"features/add-start-date-rest-api#3"},"severity":"UNKNOWN","subscribed":true,"moved_to_id":null,"imported":false,"imported_from":"none","service_desk_reply_to":null,"epic_iid":null,"epic":null,"iteration":null,"health_status":null}%

# Update an issue with a start date
❯ curl --request PUT \
  --header "PRIVATE-TOKEN: $GDK_PAT" \
  --url "http://gdk.test:3000/api/v4/projects/37/issues/3" \
  --data "start_date=2025-02-01"
{"id":723,"iid":3,"project_id":37,"title":"New feature","description":null,"state":"opened","created_at":"2026-05-27T01:24:05.041Z","updated_at":"2026-05-27T01:25:27.514Z","closed_at":null,"closed_by":null,"labels":[],"milestone":null,"assignees":[],"author":{"id":1,"username":"root","public_email":null,"name":"Administrator","state":"active","locked":false,"avatar_url":"https://www.gravatar.com/avatar/15bbd3f9e9852c9d6f0e82619c57f34c26004189add66e481c8524d6394ea445?s=80\u0026d=identicon","web_url":"http://gdk.test:3000/root"},"type":"ISSUE","assignee":null,"user_notes_count":0,"merge_requests_count":0,"upvotes":0,"downvotes":0,"start_date":"2025-02-01","due_date":null,"confidential":false,"discussion_locked":null,"issue_type":"issue","web_url":"http://gdk.test:3000/features/add-start-date-rest-api/-/work_items/3","time_stats":{"time_estimate":0,"total_time_spent":0,"human_time_estimate":null,"human_total_time_spent":null},"task_completion_status":{"count":0,"completed_count":0},"weight":null,"blocking_issues_count":0,"has_tasks":true,"task_status":"","_links":{"self":"http://gdk.test:3000/api/v4/projects/37/issues/3","notes":"http://gdk.test:3000/api/v4/projects/37/issues/3/notes","award_emoji":"http://gdk.test:3000/api/v4/projects/37/issues/3/award_emoji","project":"http://gdk.test:3000/api/v4/projects/37","closed_as_duplicate_of":null},"references":{"short":"#3","relative":"#3","full":"features/add-start-date-rest-api#3"},"severity":"UNKNOWN","subscribed":true,"moved_to_id":null,"imported":false,"imported_from":"none","service_desk_reply_to":null,"epic_iid":null,"epic":null,"iteration":null,"health_status":null}%

# Update an issue without params
# This shows that start_date is one of the parameters must be provided
❯ curl --request PUT \
  --header "PRIVATE-TOKEN: $GDK_PAT" \
  --url "http://gdk.test:3000/api/v4/projects/37/issues/3"
{"error":"assignee_id, assignee_ids, confidential, created_at, description, discussion_locked, due_date, start_date, labels, add_labels, remove_labels, milestone_id, milestone, state_event, title, issue_type, weight, epic_id, epic_iid are missing, at least one parameter must be provided"}%

How to set up and validate locally

1. Switch to the feature/add-start-date-to-issue-rest-api branch.
2. Use the curl commands in the Screenshots or screen recordings section to validate that it works. The response should include the correct start_date that was initially passed in.

MR acceptance checklist

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